Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<p>Click the button, and display "green" if a boolean value is <em>true</em>, otherwise display "red".</p>

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

<script>
Boolean.prototype.myColor = function() {
    if (this.valueOf() == true) {
        return "green";
    } else {
        return "red";
    }
};	

function myFunction() {
    var a = true;
    document.getElementById("demo").innerHTML = a.myColor();
}
</script>

</body>
</html>